This invention relates to seals for slotted transducers and more particularly to seals for transducers which have one or more slots parallel to the axis of a cylindrical transducer to provide a watertight seal to the interior portion of the cylindrical transducer. The types of transducers to which this invention is particularly applicable may be characterized as the split-ring or split-cylinder type of transducer having a single slot parallel to the transducer axis and the slotted-ring or slotted-cylinder type of transducer which has more than one slot parallel to the transducer axis. These latter transducers are also characterized as multi-slotted rings or multi-slotted cylinders. All these transducers are grouped together because, as viewed externally, they all have a slotted cylindrical (or near cylindrical) appearance and will be referred to herein as split-ring transducers.
These prior art electrical to acoustic split-ring transducers are driven by the variety of means which include natural piezoelectrics (e.g. quartz), synthetic piezoelectrics (e.g. a ceramic), magnetostriction, variable reluctance (a magnetic drive), and moving coil drives. The various drive mechanisms are well known in the art of acoustic transducer design and may be utilized with the transducers which incorporate this invention. Typical embodiments of prior art split-ring transducers are found in W. T. Harris, U.S. Pat. No. 2,812,452 issued Nov. 5, 1957, and as modified in H. W. Kompanek, U.S. Pat. No. 4,651,044, issued Mar. 17, 1987.
For the prior art fluid-filled transducer 10 shown in end view in FIG. 1, the shell 11 is shown with a gap 14 through which the fluid 13 in which the transducer 10 operates exits and enters the interior 16 of the transducer 10. The interior of the split-ring transducer may be either fluid-filled, or fluid-filled with a pressure release device such as an air- or gas-filled bladder 18 within the interior of the split-ring transducer. The direction arrows 19 illustrate the direction of water flow when the transducer 10 has its drive member 12 (shown as a segmented ceramic drive) electrically energized to cause the shell 11 to expand. The exterior surface 15 of shell 11 moves the fluid in the direction 19 thereby producing the exterior compressive sound field. At the same time, a rarefield sound field is created within water 13 in the interior 16 of transducer 10. Two defects exist with this transducer. First, the interior and exterior sound fields (which are out-of-phase) destructively interfere with each other when the interior waves escape through slot 14 or through the ends of the cylinder. This causes a reduction in sound power output. Second, the fluid mass that rushes to-and-fro within slot 14, and at the ends of the cylinder as well, tend to raise the mechanical Q of the transducer. This limits the transducer operating band because the width of the resonance peak is very sharp. There are also attendant viscous mechanical losses which occur with fluid flow at slot 14 and cylinder ends, which further reduces sound power output.
The interior and exterior out-of-phase acoustic radiation problem also exists in a free-flooded transducer ring as shown in FIG. 2 where the acoustic transducer 20 is suspended by support member 27 at its nodal point 17 to the interior of an oil 21 filled enclosure 22 which is immersed in water 13. Enclosure 22 is typically an open-mesh metal cage 23 with a watertight rubber cover 24. An advantage of having the transducer 20 immersed in an oil-filled container is the electrical insulation provided by the oil which makes the construction of the transducer electrical components substantially more simple. The transducer 20 is also protected from marine organisms which may exist in the water environment. The free-flooded ring transducer 20 suffers from serious degradation in acoustic output because of oil flow into and out of its interior 16 as shown by flow direction arrows 19 through slot 14 and arrows 25 through open ends 26. Transducer 20 assembly 28 does have the advantage of eliminating a potentially severe hydrostatic pressure problem because equal fluid pressure is always maintained on the interior and the exterior of the transducer.
Fluid-filled transducers frequently use a bladder 18 as shown in FIGS. 1 and 2 to provide a pressure-release volume on the interior of the split-cylinder transducers 10, 20. The pressure release bladder 18 minimizes the effects of the out-of-phase cancellation of the interior to exterior sound field. Use of the gas-filled bladder does, however, introduce problems which limit its usefulness.
A partial solution to the acoustic radiation cancellation problem is to completely seal the interior 16 of the transducer 30 as shown in cross-sectional view in FIG. 3 with a rubber seal or boot 31 which covers the slot 14 and shell 11 ends (not shown) to provide a water seal to prevent water from entering the gas 35 (preferably sulfur hexaflouride) or air-filled interior 16 of the transducer 30. The exterior surface 32 of the boot 31 is in contact with water 13 when the transducer 30 is in operation. The large discontinuity in acoustic impedance at the air 35-water 13 interface provided by boot 31 at slot 14 significantly prevents acoustic waves from escaping from the interior 16 to the exterior 33 and in turn destructively influencing the desired exterior sound radiation. The boot 31 can be made from any suitable material or materials which can withstand the rigors of immersion in deep ocean water and can withstand the hydrostatic and dynamic-drive loads imposed upon it. The boot 31 is shown slightly depressed into slot 14 as it would be when the transducer is immersed in water. The tension produced at the ends 11' of shell 11 by boot 31 at slot 14 significantly changes the resonance frequency with change of water depth and also reduces the bandwidth of the resonance, both of which create operational difficulties since the frequency of the electrical power transmitter should be the same as the resonance frequency of the transducer for maximum power from the transducer.